Autophagy is an evolutionarily conserved mechanism of lysosomal proteolysis that is characterized by the formation of double-membraned vesicles (autophagosomes) that envelop bulk cellular material and/or organelles. Autophagosomes subsequently fuse with lysosomes and the degradation of their cargo is mediated by lysosomal proteases. Autophagy is used for the turnover of organelles and proteins with long half-lives and also functions to generate alternative sources of metabolic fuel via nutrient recycling under stress conditions. Within the context of cancer, preclinical studies conducted with an Akt-driven tumor model established that autophagy is preferentially activated in malignant cells during the early stages of tumorigenesis. Although multiple subsequent studies have confirmed that autophagy functions as mechanism of tumor suppression via the elimination of defective pre-malignant cells, overwhelming evidence supports a major role for autophagic degradation in the maintenance of bioenergetic homeostasis under stress conditions including hypoxia and nutrient deprivation. Additionally, autophagy has emerged as an important mechanism of resistance to radiation, classical chemotherapy, and targeted anticancer agents due to its ability to augment the survival capacity of malignant cells. Many recent studies have shown that pharmacological or genetic inhibition of autophagy significantly enhances the efficacy of therapeutic agents. Carew et al., Cancer Manag Res.: 4:357-365 (2012). These collective findings demonstrate that inhibition of autophagy is a promising strategy with broad potential applications.
Chloroquine (CQ) and hydroxychloroquine (HCQ) have been used for decades to treat malaria, rheumatoid arthritis, and lupus and represent two of a very small group of FDA-approved drugs that disrupt lysosomal function and consequently inhibit autophagy. Currently, CQ and HCQ are the best studied FDA-approved drugs that are known to inhibit autophagy. More than 30 phase I and II clinical trials utilizing CQ or HCQ in combination with anticancer agents have been initiated during the last 6 years. However; the clinical applications of CQ/HCQ with respect to autophagy inhibition may ultimately be limited due to their known ability to induce ocular toxicity and the possibility that they may not completely disrupt autophagy at doses that are safe and well tolerated. New autophagy inhibitors are desperately needed. LUC (Miracil D) is a thioxanthone drug that crosses the blood-brain barrier and has been extensively used as an anti-schistome agent. Del Rowe et al., Int J Radiat Oncol Biol Phys.: 43(1): 89-93 (1999). The drug also blocks topoisomerase II activity and has been reported to inhibit AP endonuclease (APE1), an important enzyme in DNA base excision repair. Based on these properties, LUC is currently being investigated as a sensitizer to chemotherapy and radiation. The inventors recently discovered a novel mechanism of action for lucanthone that is characterized by the disruption of lysosomal function, inhibition of autophagy, and induction of apoptosis (U.S. Pat. No. 8,524,762). Similar to CQ, these effects enable LUC to potentiate the efficacy of other standard of care agents. LUC-induced apoptosis occurred through a p53-independent mechanism and notably, LUC displayed more potent anticancer activity (approximately 10× greater) compared to CQ. Carew et al., J Biol Chem.; 286(8): 6602-6613 (2011). Despite these reported properties, to date, no comprehensive structure-activity relationship (SAR) studies have been performed to optimize and illuminate the autophagic inhibition triggered by LUC or CQ/HCQ.